A chain-type bike lock, made of mostly magnesium instead of steel. Beneath the magnesium layer is a substance tha burns on contact with air. On top of the magnesium layer is a quick-burning layer to get the whole chain burning at once. If a thief cuts the lock, the inner layer ignites the magnesium,
which melts the bike into worthless scrap. Thus, there's no reason for a thief to cut the lock and try to steal the bike.

It'll hurt people without discretion. What if it's on the bike while you get into an accident? While you could have survived with a broken leg, now you'll have to amputate it. (I don't care about the person's car, but it'll hurt the most the one who's not surrounded by a cage of steel.)